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This noncompliant code example is configured as setuid-superuser. It calls bind() and later forks out a child to perform the bookkeeping tasks. The program continues to run with superuser privileges even after the bind() operation has been carried out.
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int establish(void) {
struct sockaddr_in sa; /* listening socket's address */
int s; /* listening socket */
/* Fill up the structure with address and port number */
sa.sin_port = htons(portnum);
/* Other system calls like socket() */
if (bind(s, (struct sockaddr *)&sa,
sizeof(struct sockaddr_in)) < 0) {
/* Perform cleanup */
}
/* Return */
}
int main(void) {
int s = establish();
/* Block with accept() until a client connects */
switch (fork()) {
case -1 : /* Error, clean up and quit */
case 0 : /* This is the child, handle the client */
default : /* This is the parent, continue blocking */
}
}
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The program must follow the principle of least privilege while carefully separating the binding and bookkeeping tasks. To minimize the chance of a flaw in the program from compromising the superuser-level account, it should drop superuser privileges as soon as the privileged operations are completed. In the code shown below, privileges are permanently dropped as soon as the bind() operation is carried out. The code also ensures privileges may not be regained after being permanently dropped, as in rule POS37-C. Ensure that privilege relinquishment is successful.
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/* Code with elevated privileges */
int establish(void) {
struct sockaddr_in sa; /* listening socket's address */
int s; /* listening socket */
/* Fill up the structure with address and port number */
sa.sin_port = htons(portnum);
/* Other system calls like socket() */
if (bind(s, (struct sockaddr *)&sa,
sizeof(struct sockaddr_in)) < 0) {
/* Perform cleanup */
}
/* Return */
}
int main(void) {
int s = establish();
/* Drop privileges permanently */
if (setuid(getuid()) == -1) {
/* Handle the error */
}
if (setuid(0) != -1) {
/* Privileges can be restored, handle error */
}
/* Block with accept() until a client connects */
switch (fork()) {
case -1: /* Error, clean up and quit */
case 0: /* Close all open file descriptors
* This is the child, handle the client
*/
default: /* This is the parent, continue blocking */
}
}
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